1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus which have the electrophotographic photosensitive member.
2. Related Background Art
In recent years, in electrophotographic apparatus such as copying machines and printers, widely used is an electrophotographic photosensitive member (an organic electrophotographic photosensitive member) having a photosensitive layer containing an organic charge-generating material and a charge-transporting material. As such a photosensitive layer, from the viewpoint of durability, what is prevalent is one having layer configuration of a multi-layer type (regular-layer type) in which a charge generation layer containing a charge-generating material and a charge transport layer (a hole transport layer) containing a charge-transporting material are superposed in this order from the support side.
Of charge-generating materials, a charge-generating material having sensitivity in the red or infrared region is used in electrophotographic photosensitive members mounted to laser beam printers or the like having markedly advanced in recent years, and the demand therefor has increased with more frequency. As charge-generating materials having a high sensitivity in the red or infrared region, phthalocyanine pigments such as oxytitanium phthalocyanine, hydroxygallium phthalocyanine and chlorogallium phthalocyanine and azo pigments such as monoazo, bisazo and trisazo pigments are known in the art.
There, however, has been a problem that, where such highly sensitive charge-generating materials are used, electric charges are generated in so large a quantity that electrons existing after holes have been injected into the hole transport layer tend to stagnate in the charge generation layer to tend to cause memory. Stated specifically, what is called a positive ghost, in which the image density comes high only at areas exposed to light at previous rotation, and what is called a negative ghost, in which the image density comes low only at areas exposed to light at previous rotation, are seen in images reproduced.
As background art which can keep such a ghost phenomenon from occurring, Japanese Patent Applications Laid-open No. H11-172142 and No. 2002-091039 disclose techniques in which II-type chlorogallium phthalocyanine is used as the charge-generating material. Japanese Patent Application Laid-open No. H07-104495 discloses a technique in which a charge generation layer making use of oxytitanium phthalocyanine is incorporated with an acceptor compound. Japanese Patent Applications Laid-open No. 2000-292946 and No. 2002-296817 disclose techniques in which a charge generation layer making use of a phthalocyanine is incorporated with a dithiobenzyl compound. Besides, Japanese Patent Applications Laid-open No. H02-136860, No. H02-136861, No. H02-146048, No. H02-146049, No. H02-146050, No. H05-150498, No. H06-313974, No. 2000-039730, No. 2000-292946 and No. 2002-296817 disclose techniques in which the charge generation layer is incorporated with an electron-transporting material, an electron-accepting material or an electron-attracting material.
Incidentally, Japanese Patent Application Laid-open No. 2001-040237 discloses a technique in which, for the purpose of making sensitivity higher, an organic acceptor compound is added in the step of pigmentation to produce phthalocyanine crystals.
Electrophotographic techniques have made remarkable progress in these days, and electrophotographic photosensitive members are also required to have much superior performance.
For example, black and white images such as characters or letters have been main in the past. In recent years, however, there is an increasing demand for color images of photographs or the like, and the requirement for their image quality is becoming higher year after year.
The above ghost phenomenon tends to appear especially in halftone images, and especially come into important question in color images, which are often formed by superimposing halftone images.
In addition, in the case of color images, even though the level of a ghost for each color is equal to that of black and white images, the ghost phenomenon tends to appear conspicuously because a plurality of colors are superimposed.
As a method for keeping the ghost phenomenon from occurring, a method is available in which the electrophotographic apparatus is provided with a destaticizing means such as pre-exposure. However, from the viewpoint of making the electrophotographic apparatus main body low-cost and small-size, it has become frequent to provide no destaticizing means.
The above background art has not been sayable to be well effective for such circumstances that are severe on the ghost phenomenon.